The present invention relates to Fluid Catalytic Cracking (FCC). It discloses both a process and an apparatus for an improved FCC. Due to the increasing cost of crude oil, refineries are under ever increasing pressure to economise their operations and to look for novel and innovative approaches to achieve savings. In the prior art, FCC was done with feedstocks that had a comparatively lower boiling temperature because feedstock having high boiling temperatures are considered unsafe. Also, the catalysts faced much higher degree of de-activation as the feedstock boiling temperature went up. The present invention addresses this problem and offers a process and apparatus which makes it possible to use feedstock with higher boiling temperatures. This big advantage and affords considerable savings in FCC. It presents a multiriser resid catalytic cracking apparatus with catalysts and adsorbent regenerators and a process to achieve the same.
Cracking is fundamentally a high temperature treatment which breaks up heavier or large hydrocarbon molecules into small ones, often at the same time altering their internal construction. Fluid Catalytic Cracking (FCC) is one of the most important processes employed in petroleum refineries for catalytic cracking of hydrocarbons, particularly petroleum fractions such as gas oils, to lower molecular weight gasoline and fuel oil products. This process is practiced industrially in a cyclic mode wherein hydrocarbon feedstock is contacted with hot, active solid particulate catalyst without added hydrogen at rather low pressures of up to about 100 psig and at temperatures sufficient to support the desired cracking.
Typical FCC feedstocks are distillates of vacuum tower having a boiling range from 300° C. to 550° C. The feedstocks boiling in excess of about 550° C., typically vacuum and atmospheric bottoms containing higher amount of Conradson Carbon Residue (CCR) are undesirable as FCC feedstocks due to presence of more organic metal compounds, such as nickel, vanadium porphyrins along with sodium and basic nitrogen compounds. However, refiners are showing interest for processing of heavy residues in the FCC Units for enhanced conversion and to improve refinery margin. These metals cause many undesirable reactions during cracking of heavy oils, specifically nickel and vanadium, are quite harmful to the FCC catalysts. These metals deposit on the catalyst and accumulate with time. Metals such as Ni and V are poisons in the FCC process, reducing gasoline yields and increasing production of undesirable dry gas such as hydrogen, methane and coke yields, decreasing the selectivity of the catalyst in making liquid products.
The high CCR of the feed tends to form coke on the catalyst surface, which in turn brings down the catalyst activity and its selectivity. Moreover, the higher deposit of coke on the catalyst increases the regenerator temperature and therefore catalyst/oil ratio reduces to maintain the heat balance of FCC unit. The FCC catalyst can tolerate a maximum temperature of up to 750° C., which limits the CCR of feed that can be processed in a FCC unit. At present, FCC unit with two stage regenerators with catalyst coolers can handle feed CCR up to 8 wt % economically.
These problems are understood and recognized in the prior art and various methods, such as delayed coking, residue hydro demetallation and desulfurization processes have been proposed to upgrade the feedstock containing more CCR and metals. However, these processes are used for feedstock preparation and require other secondary processes for producing lighter products. In the past, there have been some efforts to passivate the damaging effects of nickel and vanadium on the catalyst. These efforts have resulted only with some success in the passivation of nickel. Thus, by the known methods, it is presently possible to handle up to 30 PPM of nickel on the feed and up to 10,000 PPM nickel on the equilibrium catalyst. Similarly, with the known processes, vanadium up to only 30 PPM on feed and 15000 PPM on the equilibrium catalyst can be handled economically. These above limits pose serious problem of residue processing capability of FCC units. As such, huge quantity of metal laden equilibrium catalyst is withdrawn from resid FCC/RFCC units to keep the circulating catalyst metal level within the tolerable limit. Passivation changes the heat balance of the unit and product yield pattern, re-stabilize the unit at another steady state. The changed heat balance also calls for a change in operating conditions such as lower regenerator temperature, increased catalyst circulation, etc. As regards the passivation of basic nitrogen compounds, suitable passivation technology is yet to be found.
In addition to the developments of passivation technologies, there have been some important design changes made in FCC for efficient residue processing. One such design change is the two-stage regeneration in place of single stage regeneration. The advantage of two-stage regenerator is flexibility to handle additional feed CCR without requiring catalyst cooler. However, even with two-stage regenerator without catalyst cooler, there is limitation to increase feed CCR above 4-5 wt % and vanadium above 15-20 PPM on feed.
U.S. Pat. No. 5,324,417 relates to fluid catalytic cracking and cleanup of waste streams such as slop oils, spent caustic, spent DEA, spent activated carbon, spent resins, refinery sludges and the like. This patent describes a method for simultaneous conversion of contaminated streams such as refinery slop, sludge oils, etc. and vacuum gas oils in an auxiliary reactor and conventional FCC unit respectively. Auxiliary reactor is a bubbling fluidized bed reactor, which is isolated from the main FCC reactor. Part of FCC E-cat, removed from the main FCC reactor is used in an auxiliary reactor but share a product fractionator and its downstream with the main FCC reactor. Regeneration of the spent catalyst of main FCC reactor and auxiliary reactor are carried out separately. The process is capital cost intensive as it uses two independent FCC units.
U.S. Pat. No. 5,919,352 and PCT Application No. 99/03951 explains a method for conversion of residue to lighter products in two stage process wherein the first stage is an upgrading stage by means of thermal cracking in a horizontal moving bed of fluidized hot particles wherein the CCR content and metals content of residual feedstock is lowered and the second stage is a catalytic cracking stage containing a reactor, regenerator for conversion of upgraded residue to lighter products.
U.S. Pat. No. 5,059,302 discloses an improvement in the fluid catalytic cracking (FCC) of hydrocarbon feedstocks, especially those containing one or more impurities, such as metals, basic nitrogen compounds and asphaltenes (Conradson carbon), in which a particulate fluidizable material that is a sorbent is used to remove one or more of such impurities from the feedstock before the feedstock contacts particles of cracking catalyst for conversion of the feedstock into lighter products, such as gasoline. The separation of the sorbent and catalyst particles is carried out in the regenerator.
It has been suggested in the art to use active adsorbent to capture the metals from the feed before the feed is contacted with the active catalyst in the same riser. It has also been suggested in the art to use a separable mixture of catalyst and demetallizing additive particles. For example, in U.S. Pat. Nos. 4,895,637, 5,021,222, 5,110,775 and PCT Application No. 00/01484 suggest a physically separable mixture of FCC catalyst and demetallizing additive having sufficient differences in their settling velocities so that the particles are separable in a single stage regenerator. U.S. Pat. No. 5,110,775 also suggests providing a separate vessel, outside the regenerator to improve the regeneration of the additive particle. Though such a process is simple, there are several practical disadvantages, which limit its resid-handling capability, poor segregation efficiency due to sufficient turbulence and mixing in the bed and as vanadium is highly mobile in the regenerator atmosphere and the vanadium may escape from the demetallizing additive to the catalyst particle at these conditions. This defeats the basic purpose of eliminating catalyst deactivation due to metal poisoning.
U.S. Pat. No. 5,286,691 also describes a process for converting a residual hydrocarbons into lighter products using separable metal getter and cracking catalyst particles. Single regenerator is used for regeneration of both type of particles and demetallation is carried out using a demetallation solution in a separate in vessel provided outside of the regenerator.
U.S. Pat. Nos. 4,875,994, 5,059,302 and 5,196,172 explain the use of two-stage regenerator and physically separable particles for conversion of residual oils having large amounts of metals and CCR. Particle separation is carried in the first regenerator based on the size variation of the particles. However, this process also does not eliminate catalyst deactivation due to metal poisoning as additive and catalyst are exposed to combustion conditions in regenerator-1, wherein separation of these two particles is taking place.
Another approach to process residual oils in single riser reactor is explained in U.S. Pat. Nos. 6,149,875, 6,656,344, 7008595 and 7381322. It suggests using physically separable mixture of catalyst and adsorbent for having sufficient differences in their minimum fluidization velocities and or settling velocities depending upon the nature of catalyst. The mixture of adsorbent and catalyst can be separated either in stripper using difference in their minimum fluidization velocities or in a separate vessel using difference in their settling velocities. Deactivation of the catalyst particles due to metals poisoning is eliminated, as these solid particles are separated in non-combustion environment. The disadvantage of these processes is catalyst activity dilution due to presence of adsorbent particles while cracking of the cleaner feedstock at elevated portion of the riser.
Therefore, there is a need to upgrade residual oils containing higher CCR and metal concentrations to lighter products through a more efficient and cost effective method. There is also a need to develop a process to upgrade residue feed stocks while extending cracking cycle life of the catalyst and the improved yield of lighter compounds.
The present invention is aimed at avoiding or overcoming the difficulties or limitations encountered in the prior art to provide an improved resid cracking process and an apparatus for converting residual oil containing high concentrations of Conradson Carbon Residue (CCR), poisonous metals such as vanadium, nickel and sodium; basic nitrogen and sulphur compounds impurities into substantially impurity free lighter products by employing an adsorbent to remove one or more impurities from the feedstock before the feedstock comes in contact with the cracking catalyst whereby the cracking cycle life of the catalyst is extended and the yield of lighter products is improved.
Another advantage of the invention is to provide an improved process and apparatus comprising multiple riser reactors wherein the feedstock impurities are removed by contacting the feed with an adsorbent in a first reactor while main cracking of impurity free feedstock is done in another reactor to obtain high value products such as propylene, LPG, gasoline etc. thus eliminating the catalyst deactivation due to metal impurities and FCC catalyst dilution effect to achieve a better conversion and higher catalyst longevity.
Still another advantage of the invention is to provide a resid cracking apparatus with stripper cum separator that handles the differences in the particle size and density of adsorbent and catalyst particulates for separation.
Another advantage of the invention is to provide a resid cracking apparatus which uses a concept of vertical separator plate inside the stripper cum separator vessel to restrict mixing of the adsorbent and the catalyst used therein eliminating the requirement of physical differences between the adsorbent and the catalyst for the separation.
Still another advantage of the invention is to provide separate regenerators for regenerations of adsorbent having deposits of feed impurities and spent cracking catalyst.
Yet, another advantage of the invention is to provide a compatible design for said apparatus so that it can be used also with other FCC designs.
Still another advantage of the invention is to enhance the life of the apparatus by controlling the operating temperatures optionally by using catalyst coolers.